Over the years, mass spectrometry has been the analytical method of choice used in accurate and sensitive determination of the atomic or molecular composition of a sample. Time-of-flight (TOF) mass spectrometers have been one type of apparatus used for mass spectrometry. In recent years, the applications and uses for the time-of-flight mass spectrometer have increased considerably as compared to other methods. The salient characteristics of this type of mass spectrometer are its high sensitivity due to the near unity transmission of the ions through the system and the near simultaneous detection of all masses thereby utilizing all created ions. A drawback for these instruments has been the relatively poor resolution or mass discrimination due to the energy spread (which leads to time smearing) of species extracted from the ion source region. The energy spread is due principally to two effects: (1) the finite size of the ionization region introduces an energy spread because ions are extracted from regions of different potential, and (2) the initial velocity spread of the neutral atoms before ionization (a gas being generally used as the sample).
An improved version of the TOF mass spectrometer (U.S. Pat. No. 2,685,035) was first described by W. C. Wiley and L. H. McLaren in Rev. Sci. Instr., 26, 1150 (1955). This instrument of Wiley, et al., utilizes a two-step acceleration process or a time delay between ionization and application of extraction fields, or a combination of both to achieve time focusing and thereby improve the resolution of the instrument. By adjustment of operating parameters, the instrument can be optimized to partially compensate for effect (1) or effect (2) above, but not both simultaneously.
A different system, described by B. A. Mamyrin, V. I. Karataev, D. V. Shmikk, and V. A. Zagulin, Sov. Phys. JEPT, 37, 45 (1973) utilizes a time delay introduced by the reflection of ions in a potential barrier to compensate for the differing energies of the extracted ions. More recent workers have improved resolution by utilizing the above techniques in combination with procedures of reducing the velocity spread of the neutral atoms before ionization. The utilization of supersonic molecular beam techniques can significantly reduce the velocity spread of the atoms in the direction parallel to the flight path. These variations are described by D. M. Lubman and R. M. Jordon, Rev. Sci. Instr., 56, 375 (1985); R. B. Opral, K. G. Owens and J. B. Reilly, Anal. Chem. 57, 1884 (1985).
All of the above techniques are applicable to samples that are already in the gas phase, but do not address the situation in which the atoms are removed from a solid surface. In many instances the velocity spread of the removed atoms (from a solid) is intrinsic to the removal method and cannot be reduced. Furthermore, the systems for achieving compensation are elaborate and often defeat the general advantages of TOF mass spectrometers.
Accordingly, it is a principal object of the present invention to provide an improved time-of-flight mass spectrometer, and method of its operation, which compensates for the energy spread that causes poor resolution.
It is another object of the present invention to provide a time-of-flight mass spectrometer where the material is first removed from a surface and wherein energy spread is compensated for.
It is a further object of the present invention to provide a mass spectrometry method using a time-of-flight mass spectrometer wherein atoms or molecules are removed from a surface at a well defined time, and ionized at a well defined time after a selected time delay.
These and other objects of the present invention will become apparent upon a consideration of the following drawings and a complete description thereof.